In wireless communication networks, recent developments of the 3GPP Long Term Evolution, LTE, facilitate accessing local IP based services in the home, the office, in public hot spot or even in outdoor environments. One area in which the access and local connectivity of these local IP based services may be used is in the direct communication between wireless devices in the close proximity of each other. In this case, close proximity may typically refer to less than a few tens of meters, but sometimes even up to a few hundred meters.
This direct mode or device-to-device, D2D, communication may demonstrate a number of potential gains over traditional cellular communication. This is because D2D devices are much closer to one another than other cellular devices that have to communicate via cellular access point, e.g. a radio network node such as an eNodeB.
One of these potential gains is capacity. Radio resources, such as, e.g. Orthogonal Frequency-Division Multiplexing, OFDM, resource blocks, between the D2D and cellular layers may be reused, resulting in reuse gains. Also, the D2D link uses a single hop between the transmitter and receiver points as opposed to the double-hop link via a cellular access point, resulting in hop gains.
Another potential gain is peak rate. Because of the proximity, and potentially favorable propagation conditions for the D2D link, high peak rates are possible to achieve, resulting in proximity gains.
A further potential gain is latency. When wireless devices communicate over a direct D2D link, forwarding via the cellular access point is short cut and the end-to-end latency between the wireless device may decrease.
In these mixed cellular and D2D wireless communication networks, it has been suggested to locate D2D communication on cellular uplink, UL, resources in a way such that Time-Division Duplex, TDD, is the duplex transmission scheme of the D2D communication. This means that the cellular UL resources would be allocated for D2D communication transmission in both upstream and downstream directions for each D2D pair of wireless devices in a Time-Division Multiplexed, TDM, manner.
As a consequence of this configuration, and as one of the key components of the physical and Medium Access Control, PHY/MAC, layer, a HARQ mechanism for the D2D communication has to be designed to take the following aspects into account.
First, HARQ transmission is coupled with a TDD configuration that is specific for the D2D communication. This means that the HARQ timing must fit for the chosen TDD configuration.
Secondly, on the one hand, the D2D TDD configuration is specific for each D2D pair. This means that each D2D pair will need to have its own HARQ timing and find the Dedicated Control CHannel Acknowledgement/Non-Acknowledgement, DCCH A/N, resource locally. This may e.g. be performed based on the Control Channel Element, CCE, index n_cce and the subframe index m according to 3GPP TS 36.213 E-UTRA Physical layer procedures 2011.03.
On the other hand, the DCCH resource allocation should be controlled from a network point of view in order to ensure that the resources are efficiently used. This may be referred to as network-assisted D2D communication.
Thirdly, the amount and location of available cellular UL resources for D2D transmission, i.e. D2D-compatible subframes in which D2D communication is allowed to be scheduled, would also be load dependent and thus time-varying. This will affect the amount or delay of the DCCH that is carrying the A/N for the D20 communication, i.e. a D2D control channel which is similar to the Physical Uplink Control CHannel A/N, PUCCH A/N, in LTE. Thus, it will also affect the HARQ timing.
In view of the above, there are numerous requirements to be considered when attempting to provide a HARQ transmission for a D2D communication between wireless devices.
US 20120163252A1 relates to the problem of HARQ mechanism in D2D communication. Here, the ratio of the upstream and down-stream cellular UL resources for the D2D link is fixed to a 1:1 ratio. This limits the D2D communication to only apply to a single case, and thus is too limited to be applied to different upstream and downstream ratio scenarios in D2D communication.